FLUTTER INTERVIEW QUESTIONS

What are ??, ??=, ?., …? in Dart?

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Looking weird, don’t they?
Those are null-aware operators in Dart and they tend to shorten your code a lot.

??

Called also null operator. This operator returns expression on its left, except if it is null, and if so, it returns right expression:

void main() {
  print(0 ?? 1);  // <- 0
  print(1 ?? null);  // <- 1
  print(null ?? null); // <- null
  print(null ?? null ?? 2); // <- 2
}

??=

Called also null-aware assignment. This operator assigns value to the variable on its left, only if that variable is currently null:

void main() {
  int value;
  print(value); // <- null
  value ??= 5;
  print(value); // <- 5, changed from null
  value ??= 6;
  print(value); // <- 5, no change
}

?.

Called also null-aware access(method invocation). This operator prevents you from crashing your app by trying to access a property or a method of an object that might be null:

void main() {
  String value; // <- value is null
  print(value.toLowerCase()); // <- will crash
  print(value?.toLowerCase().toUpperCase());  // <- will crash
  print(value?.toLowerCase()?.toUpperCase()); // <- output is null
}

…?

Called also null-aware spread operator. This operator prevents you from adding null elements using spread operator(lets you add multiple elements into your collection):

void main() {
  List<int> list = [1, 2, 3];
  List<String> list2; // <- list2 is null
  print(['chocolate', ...?list2]); // <- [chocolate]
  print([0, ...?list2, ...list]); // <- [0, 1, 2, 3]
  print(['cake!', ...list2]);  // <- will crash
}

?

And for bonus, ? operator.
Looking similar to those we already saw, this is NOT a null-aware operator, but a ternary one. Ternary operator is used across many languages, so you should be…